RECENTDISCUSSION

Some wild populations have evolved a complex amphidromous breeding strategy in which adults live and spawn in freshwater or brackish streams and the pelagic post-hatch larvae are washed downstream to the sea where the post-larval fry spend the first part of their life developing in estuarine nursery zones under brackish to full marine conditions.

Once they reach a certain stage of development they begin to migrate upstream with some mov…

There are a number of unidentified species within the grouping of which several are referred to using a two-letter code based on their respective colour pattern for ease of reference, with ‘CB’ shorthand for ‘cross band’, for example.

R. sp. ‘CB’ is the commonest member of the grouping in Japan but has not yet entered the international aquarium trade as far as we know.

Known from various parts of the middle and upper Mekong river basins with record existing from the Nam Tha, Nam Ou, Nam Khan, Nam Lik, Nam Ngum and Nam Mang tributary systems in Laos, the Nam Noeua (a tributary of Nam Ou) in Vietnam, and the Mae Nam Kok in Thailand.

Occurrences in the Chao Phraya drainage in central Thailand most likely refer to the congener R. chiengmaiensis.

This species’ name has appeared quite regularly on ornamental fish trade lists since the mid-00’s but it’s unclear whether the species itself has ever been exported since fish labelled as such do not appear to fully match the most recent key (see below).

That said, its natural habitats lie in the same region as the loach Yaoshania pachychilus so it’s reasonable to assume that some specimens may have been collected, and the fish in our images were ca…

This species may not yet have appeared in the ornamental trade but it has been collected by a few individuals.

It can be told apart from related species from Taiwan by possessing 30-32 longitudinal (lateral) scales vs. 32-39 in other species, and 6-8 scale rows between the origin of the first dorsal-fin and upper pectoral-fin base vs. 9-15 in other species.

It’s unlikely that this species has been in the aquarium hobby given its natural range and the lack of commercial fishing for the ornamental trade in that area.

Among congeners it’s most similar to R. davidi but can be told apart by presence of 8 (vs. always 9 in R. davidi) soft dorsal-fin rays, 10 (vs. 11-12) transverse scale rows, and 6-7 (vs. 8-10) scales between the first dorsal-fin origin and upper pectoral-fin base.

This species appears to exist in a number of different forms which exhibit differences in colour pattern, morphology, or both, and it’s currently unclear whether all of them are truly conspecific or not although those in the aquarium trade all appear similar to one another.

We’ve been unable to obtain a copy of the original description so it’s not currently possible to provide a detailed diagnosis either, with most recent studi…